Loudspeaker, in particular in micro-speakers for portable devices (mobile phones), and more in particular receiver micro-speaker (also called ear-pieces, responsible for the voice sound-transmission), need thinner elements in order to reduce the overall size of the loudspeaker. In general, a loudspeaker comprises a diaphragm which is excited by a coil or another vibrating element.
In US 2013/0016874 A1 for example this function is represented by the element 121 of a diaphragm 12 which guarantees high break-up frequency and low weight. This element is often called membrane plate, to be distinguished from the surround (connecting area 123) which is often called membrane. The characteristics required by a membrane plate are:                a. High material resonance frequency—to guarantee a linearity and the absence of acoustic peaks in the hearable region        b. Low weight—to reduce the moved mass and consequently increase the sound pressure level and the efficiency of the speaker        c. High temperature resistance—to guarantee the same mechanical stiffness at higher working temperatures        
The resonance frequency of a material is directly proportional to its length and width and a figure of merit, here defined “Frequency Factor”. The frequency factor is defined as follow:
  d  ⁢            B      ρ      
Where d, is the total thickness, B is the bending module, and ρ is the density of the membrane plate material. The square root is also the speed of sound of the material.
In micro-speakers, due to very small available thickness, the membrane plates are generally designed with a total thickness lower than 500 μm.
For these applications, due to the low available thickness, in order to achieve high frequency factors, it is necessary to utilize high mechanical performance materials. Sandwich constructions represent a solution for this application, since they offer a proper ratio of bending module to weight (see also “An Introduction to Sandwich Construction”, Zenkert, D., 1995, Engineering Materials Advisory Services Ltd).
In micro-speaker applications, the actual state of the art is disclosed for example in CN 204707266 U.